Method and apparatus for continuous centrifugal separation



June 19, 1951 J. H. BOIVINET 2,557,629

ws'rnoo AND APPARATUS FOR commuous CENTRIFUGAL SEPARATION Filed April19, 1946 6 Sheets-Sheet 1 lnuenior JeanHenri. boiuind' Ailorneg June 1a,1951 O H, BOMNE 2,557,629

METHOD AND APPARATUS FOR CONTINUOUS CENTRIFUGAL SEPARATION Filed April19, 1946 6 Sheets-Sheet 2 za 7 I" 6! /I 1/ lnue'xfior Jean, Henrioivine% June 19, 195] J. H. BOIVINET 2,557,629

METHOD AND APPARATUS FOR commuous CENTRIFUGAL SEPARATION Filed April 19,1946 6 Sheets-Sheet 3 Inuejncor Jean Henn Boivinel:

June 19, 1951 J. H. BOIVINET 2,557,629

METHOD AND APPARATUS FOR CONTINUOUS CEN'I'RIF'UGAL SEPARATION FiledApril 19, 1946 e Sheets-Sheet 4 A V/ I 7 w y Invenlzor Jean HenriBoivinei:

Aitorneg June 19, 1951 J. H. BOIVINET 2,557,629

METHOD AND APPARATUS FOR CONTINUOUS CENTRIF'UGAL SEPARATION 6Sheets-Sheet 5 Filed April 19, 1946 /5/ X/ X/ W PM I K lllllllllllll I Imu a w 1... n r e O V.\O 1 w 5 U A my u n a w. J

June 19, 195] J. H. BOIVINET 2,557,629 METHOD AND APPARATUS FORCONTINUOUS CENTRIFUGAL SEPARATION Filed April 19, 1946 6 Sheets-Sheet 6Patented June 19, 1951 METHOD AND APPARATUS FOR CONTINU- OUS CENTRIFUGALSEPARATION Jean Henri Boivinet, Aix-en-Provence, France, assignor toCompagnie de Produits Chimiques et Electrometallurgiques Alais, Frogeset Camargue, Paris, France, a corporation of France Application April19, 1946, Serial No. 663,576 In France April 19, 1945 13 Claims. (Cl.233-15) The present invention relates to methods and apparatus for thecontinuous centrifugal separation of solid particles from a liquid inwhich they are dispersed, with the possibility of washing theseparticles by another liquid.

Up to the present time, centrifugal separation of solid particles from aliquid in which they are in suspension could not .be obtained incontinuous operation. As a rule, the suspension to be treated issubjected to centrifugal action and the clarified liquid is removeduntil the solid deposit reaches a given thickness on the peripheral wallof the centrifugal apparatus. The

separation operation is then stopped and the deposit is removed throughany suitable mechanical means. Finally, if the muddy deposit that hasbeen separated is to be washed, it is necessary, with apparatus of thekind, to place it again in suspension and to subject the new suspensionthus formed to further centrifugal treatments.

Apparatus of the type used for separating cream from milk, in which theseparation and extraction take place in a continuous manner, areperfectly well adapted to the separation of emulsions of two liquids.But they cannot be used for the separation of solid particles insuspension, where the densities of the suspensions are liable to varyduring the separation operation, because in this case, the equilibriumsthat are obtained 'are unstable and simultaneous evacuation of theliquid and the mud formed by the solid particles is no longer possible.

Other types of apparatus have also been proposed but, up to the presenttime, none of them has given practical results.

The object of the present invention is to provide a method and apparatusfor obviating these difiiculties and ensuring, with a continuousoperation, the separation of solid particles from a liquidin which theyare in suspension.

With this object in view. according to a'feature of my invention, thesuspension of solid particles in a liquid is treated in a centrifugalapparatus and the mud that collects at the periphery of this apparatusis driven in a centripetal direction by a stream of water or any otherliquid the pressure of which is automatically regulated in accordancewith the very op- I eration of the apparatus.

According to my invention, this regulation of the pressure of the wateror other liquid that drives the mud in the centripetal direction is as-.sured in a very simple manner by feeding said liquid from the centertowardthe periphery through a suitable supply conduit, while the mudcarried along by said liquid is evacuated from the periphery toward thecenter through a suitable evacuation conduit. With a suitable choice ofthe respective dimensions of the various parts of the apparatus, thelevel of said liquid in the feed conduit establishes itselfautomatically at a distance from the axis of rotation such that there isalways suflicient pressure for driving along the heavy elements,whatever .be its variations resulting from variations of the percentageof heavy elements, of densities,'viscosities, and so on. The rate offlow of the liquid stream maintains itself always at the selected valueand the flow of the heavy elements is thus ensured, because unstabilityof the equilibrium is automatically corrected.

With this arrangement, the discharge orifices, which may be eithercalibrated orifices or ordinary orifices, are located far enough fromthe periphery so that they can have a relatively large cross-sectionand, hence, not be liable to p ng.

Finally, the relative rate of flow of the auxiliary liquid, that is tosay of the liquid for carrying oil the heavy particles, may be made suchthat a portionof this liquid flows (ascends) outside the evacuationconduit into the centrifugal bowl, and in countercurrent fashion withrespect to the movement of the heavy elements which flow toward theperiphery. The deposit is thus washed in the best possible conditions,if a certain dilution of the evacuated clarified liquid is notobjectionable.

The method according to the present invention further permits ofreplacing, in a suspension, the dispersing liquid by another one.

According to another feature of my invention, I

the method and apparatus for the continuous centrifugal separation ofsolid particles from the liquid in which they are dispersed, as abovesetforth, is applied to the separation of red mu in suspension in solutionsof sodium aluminate, obtained in the alkaline treatment of bauxite inautoclaves, which separation is to be carried out during themanufacture'of alumina according to the Bayer process.

In actual practice, this operation is nearly always carried out byapplying continuous decantation by the mere action of gravity after athe suspension obtained suitable dilution of from the autoclave.Methodical washing of the thickened mud collected at the bottom of thedecantation apparatus is then carried out, mostly by means of apparatusworking on the same principle. The disadvantage of such a method ofoperation is that it involves a long time of contact of the metastablesodium aluminate solutions with red mud, which constitutes, ac-

cording to my experiments. an active agent for starting decomposition.Retrogradations (precipitation), in the course of the gravitydecantation method, can be partly avoided but at the expense ofmaintaining a very high temperature in the decanting and washingapparatus. The safety limits of temperature from this point of view aregiven in a paper of G. Bauermeister and W. Fulda (Aluminium, No. 3,March 1943, pages 97-100). Thus for a concentration in caustic NaaO of130 gr. per litre, the molecular ratio of caustic NazO to A110: of thesodium aluminate solutions in equilibrium varies, as follows, as afunction of the temperature:

Temperature-89 C., 94 0., 100 C. Ratio of caustic HMO-t A1zOs-2.16,2.03, 1.78

Now, in actual practice, the temperature of a decantation apparatushardly exceeds 95 C. and the molecular ratio of caustic NazO to A110: ofthe aluminate liquors that leave the autoclave is generally close to1.8. It is therefore obvious that the solutions that are treated aremetastable and that a more or less advanced decomposition is always tobe feared. This fact is well known to the experts on alumina, who areaware that a slowing down of the decantation process is almost sure tocause a drop in the yield of the plant. This effect is particularlyserious when the treatment in the autoclave is carried out attemperatures sufllciently high for permitting the treatment of largecharges of bauxite, and the obtainment of aluminate liquors having alower caustic NazO to A120: ratio, for instance,

ranging from 1.5 to 1.6.

The application of the decantation method and apparatus according to thepresent invention to the treatment of red mud enables a reat increase inthe rate of decantation of said mud,

which considerably reduces the time in which the liquidand the red mudare in contact with each other, and also renders negligible, withrespect to the new rate of deposition, any possible variations in thedecantation aptitude of the various red muds. Finally, the applicationof thismethod further has the double advantage of making it possible toperform decantation at a much lower temperature and to make use for.

the washing operation of a much smaller proportion of water, this doubleadvantage resulting in an important economy of fuel.

The invention can be carried out with centrifugal' apparatus of allkinds. The annexed drawings show, by way of example, the application ofthe invention to the case of centrifugal apparatu comprising a containerfitted with a plurality of superposed discs or conical elements, and tothat of a centrifugal apparatus provided with a plurality of peripheralcones, but these examples should not be considered as having anylimitative character.

Preferred embodiments of the present invention will be hereinafterdescribed with reference to the accompanying drawings. given merely yway of example, and in which:

Fig. 1 is a vertical sectional view of an apparatus made accordingto afirst t of the invention;

Fig. laisadetailviewinsectionontheline II of Fig.1; Fig. 2isaviempdrtlyinelevatidnandparfly in vertical section, of a secondembodiment;

Fig.2aisaview,partlyinelevationandpartiy in vertical section, of aslight modification of the embodiment shown in Fig. 2;

Fig.3isaplanviewccrraponcling'tol 'ig.2;

Fig. 4 is a sectional view of a modification of the apparatus of Fig. 1;

Fig.5isaseetionalviewofaof the apparatus of Fig. 2;

Fig.6isaplanviewtoFlg.5;

Fig.7 isanaiualsectionalviewofacenirifugation cone;

figsisanaxialsecflonalviewoftheendofaconeofamachineaccordingtoamodiiication of that shown by Fig. '7;

Fig. 8a is a perspective view of the same arrangement as Fig. 8;

Fig. 9 is a view partially in elevation and partially in axial sectionon the line IXIX of Fig. 8;

Fig. 10 is a sectional view of the end of a cone of a centrifugalmachine, made according to still another modification.

In the embodiment illustrated by Fig. 1, the apparatus includes a mainbody I and a cover 2 fixed to said main body by means of a nut ring I 3.A plurality of conical plates it are superposed in body I. The mixtureto be decanted or separated is introduced through tube I into the spacebetween partition 9 and the bottom of body ianditisledtothebaseoftheplleof plates is on the outer side thereof.

The heavy elements or particles collect at the periphery of body i,where they meet with liquid streams of centripetal direction. Theseheavy elements are driven into and through pipes I and evacuated throughcalibrated orifices I. The mud that accumulates at the periphery of thecontainer formed by .body I and cover 2 forms funnel-shaped depositsaround pipes l, the spaces limited by these funnel-shaped mud depositsserving in the separation operation.

The liquid for driving out these heavy elements is introduced throughconduit II into distributing means where it is divided into as manyequal portions as there are devices for the evacuation of the heavyelements.

Each of these portions is led, through pipes i5, disposed betweenpartition 1 and the bottom of body I. to the points of the peripherywhere the heavy elements collect together.

The liquid in course of clarification ascends 60 between discs II andflows out in the clarified ,stafe through calibrated orifice II.

In the embodiment illustrated by Figs. 2 ands, the apparatus includes acentral body i on which are fixed a plurality of cones 2' located eitherall in the same plane or in different parallel planes.

These cones include a partition I, also of conical shape, and an axialpipe 4' leading to calibratedoriiices 5'.

The mixture to be separated or decanted is introduced through conduit 0'into distributing device I where, by means of a set of radialpartitions, it is divided into as many equal portions as there are conesI in the apparatus.

, 8 duit 8', a portion of this mixture, which is fed to a distributor 9'fitted with a perforated partition III for equalizing the rate of fiowinto cone 2' where decantation or separation is to take place.

The clarified liquid flows inwardly between distributor 9' and partition3', enters space II and escapes through calibrated orifices l2.

The heavy elements collect at the apex of the cone, where they meet witha stream of liquid of centripetal direction. These elements are driveninto pipe 4' and evacuated through calibrated orifice 5'.

The driving liquid is fed through conduit I3 into passage l5 betweencones 2' and 3', after passing through distributing device H where,owing to the action of a plurality of radial partitions, it has beendivided into as many equal portions as there are cones.

Each portion of said liquid is thus led to the apex of a cone 2', whereit is caused to flow in a centripetal direction, and thus meets theheavy elements.

In both of the embodiments above described with reference to Figs. 1 to3, I may cause a portion of the liquid of the initial suspension to flowout through conduits 4 or 4'. I may also cause a portion of theauxiliary liquid to mix with the liquid of said suspension. For thispurpose, it suffices, for' instance, either to reduce or to increase thevolume' of liquid introduced into distributing device I or H.

The distance between the various parts of centrifugal apparatus issuitably determined in such manner that the liquid level in conduit I5is normally located in the middle part of this conduit. In case ofvariation of the pressure necessary for driving along the heavyelements,this level very quickly comes to such a distance from the axis ofrevolution that the variation of pressure that results from thisvariation of position of the liquid level keeps the fiow of liquid atthe chosen rate.

In. the case of devices of the kind shown by Figs. 2 and 3, the samepressure variations take place under the effect of similar causes in theconduit 8' through which the mixture to be separated is introduced andgive the same result of keeping the rate of flow of this mixture to auniform rate.

In addition to the action exerted by the volume of the liquid and thatof the mixture introduced into the apparatus, it is further possible, byvarying the sections of orifices 5 and I2, to modify the workingconditions of said apparatus and, in particular, to act upon theproportion of water mixed with the clarified liquid or of suspensionliquid evacuated together with the heavy elements. It is thus possibleto vary within wide limits the conditions of washing of said elements.

The apparatus illustrated by Figs. 1, 2 and 3 are given merely by way ofexample, and many modifications can be made therein without, departingfrom the principle of the invention. For instance, the calibratedorifices 5' and I2 of Fig. 2 can be replaced. b tubes l6 and I1 andventholes l9 and I9 as shown in Fig. 2a. In this case, the rate of fiowis'determined by giving suitable lengths to tubes l6 and II, the latteropposing to the circulation of the elements a counter-pressure which isthe higher the nearer their orifices are from the axis of rotation a-b.The sections of these tubes should be chosen relatively large so thatonly their length need be taken into account, whatever be the rate offlow and the viscosity of the elements conveyed therethrough.

The driving liquid may itself be constituted of a suspension instead ofbeing constituted of a clear liquid. This auxiliary liquid or suspensionmay, if advantageous, be introduced under pressure, by replacing, forinstance, tube I3 of Fig. 2 by an axial tube l3 as in Fig. 1, butprovided with a stufiing box. The auxiliary liquid or suspension forcarrying along the heavy elements may be introduced by means of a pistonpump the discharge pressure of which will add its action to theautomatic action of the centrifugal apparatus, so as to overcome theresistances that are opposed to the evacuation of the heavy elements.

I may also introduce into the tube 4 or 4', in addition to the auxiliaryliquid, a gas or vapor intended to reduce the weight of the column ofsuspension to be evacuated. For the same purpose, I may also replace theauxiliary liquid or suspension that is fed through conduit l3 or l3 byan emulsion of a gas or vapor.

In the device of Fig. l,pipes 4 and I5 may be replaced by annularchannels of small width, which makes it possible todispense with theformation of funnel-shaped stable mud deposits as above set forth, thewhole of the bowl being then utilized for decantation cf the mixture.

When the density of the dispersive phase, that of the driving liquid, orthe proportion of dry material in the dispersed phase vary in the courseof a centrifuging operation, the amount of dispersive phase that isevacuated with the dry matters also varies, which may involve certaindrawbacks, in particular that of collectin a smaller amount of thedispersive phase that is employed.

The apparatus shown by Fig. 4, which is of a type analogous to that ofFig. 1, makes it possible to vary during the course of the operation thevolume of clarified liquid and, therefore, to collect the desiredquantity of this liquid.

In this apparatus, the orifices l2 for the discharge of the clarifiedliquid open into a circular channel 20 which rotates together with thebody I of the apparatus. A pipe 2| which acts as a discharge conduit andwhich does not turn together with body I, penetrates more or less intosaid channel 20 and thus permits the variation, in operation, of thelevel of the clarified liquid. and therefore the hydrodynamic stat s ofequilibrium in the apparatus, whereby it is possible to modify at willthe rate of extraction of the clarified liquid from the apparatus.

The degree of penetration of pipe 2| into channel 2|] may be variedeither manually or automatically as a function of the flow rates.

I may also arrange the heavy elements outlet orifices 5 sothat they openinto a circular channel 22, and remove solid particles mixed with thedriving liquid by means of a pipe 23 the position of which is adjustableso as to vary'the fiow rate.

Channels 20 and 22 may be used either simultaneously or separately,orifices l2 and 5 being both either calibrated orifices or uncalibrated.

The centrifuging apparatus shown by Figs. 5 and 6 is of a type analogousto that of Figs. 2 and 3. This apparatus includes a system of dischargepipes 2 I and 23 analogous to those above referred to, projecting moreor less into channels 20 and 22'' respectively, and designed to receivethe outflowing clarified liquid and the mixture of solid particles andof driving liquid.

According to my invention, the apparatus may be arranged in such mannerthat a portion of the driving liquid is mixed with the liquid of thesuspension and performs a relative washing of the mud.

- wam When washin liquid is introduced into the bottom part of adecantation chamber constituted. for instance, by the inner cone 8' ofthe apparatus of Fig. 5, this liquid produces, if its density be lowerthan that of the suspension that is being decanted, eddies that aredetrimental of the good clarification of the liquid. These eddies arethe more pronounced the greater the diil'erence of 1 density and thegreater the centrifugal field.

In order to prevent the formation of these eddies, I provide (Fig. 'l)in cone 2' a bell or funnel 31 adapted to catch the centripetal streamsof diluted suspension and to lead them, through tube 38, into the feedchamber 8' of the apparatus. With this arrangement, the portion of thedecantatlon chamber that is located between the edge of this bell orfunnel 31 and. the level of evacuation of the clarified liquid is notdisturbed by eddies and the clarification of the liquid takes placenormally.

According to another feature of my invention, it has been found that itis possible to obtain the separation of the clarified liquid withoutmixing with the driving liquid.

This result can be obtained by providing at the apexes of the cones ofthe centrifuging apparatus special devices such as those illustrated byFigs.

8 and 9.

Fig. 8 is an axial cross-sectional view of the apparatus across thecones which it comprises. Fig. 9 is a view partially in elevation andpartially in axial section of the same arrangement but through a planeperpendicular to that of Fig. 8

8 cordingtothisdeviceblockllisprovidedwitha conduit SI connected with asuitable pipe 80 andopeningintoconduitfl. Thewashingllquid can beintroduced through conduit" into conduit 82 and. therefore, into thedecanted heavy elements. The quantity and quality of the washing liquidthus introduced from the outside can be varied during operation of. thecentrifuging machine. Pipe 88 and conduit 88 may also be used for theintroductions! a gas, a vapor or an emulsion. a

In the device of Fig. 10, the conical inner partition 8 is provided, atthe apex thereof with a block 24 provided with a e 11 for the evacuationof the decanted solid particles. The

driving liquid fed through the space between i. e. along line DIIX ofFig. 8 passing through the central tube through which the muds arrive.Fig. 8a is a perspective view of the same 81'! rangement in partialcross-section through two axial planes perpendicular to one another, onebeing that of Fig. 8 and the other that of Fig. 9. According to thearrangement illustrated by these views, I provide at the interior of theapexes of the cone 1 of the centrifuging apparatus a removable block 28upon which concentric tubes 4 and 28 are fitted. The driving liquid isfed into the annular space between'these two tubes and enters conduit"provided in block 28. The decanted solid particles, accumulated at theapex of cone 2, pass through aperture 3| into conduit 32. One of theends of the conduit 88 is screwed into the wall of the cone 2.and theend of this screw forms a tight joint with the block 18. 'nie other endof the conduit 38 is connected to a circular partition, not shown,analogous to the partitions designed for the introduction of the mixtureto be decanted and for the driving liquid. In the case of acentrifugation with washing there.

are therefore three circular coaxial partitions."

A conical piece 33, made in the shape of the plug of a cock, is fittedin a fluid-tight manner in a corresponding housing formed in block 28.-It

serves to connect together the various conduits as shown by the drawingwhereby the driving liquid fedthrough conduit 88 enters conduit 84,which is connected tooutlet tube 4. is provided with a small plate Ilawhich can be made of any desired length and thickness to obtain a givenrate of flow. It is pomble, by modi- Conical piece It fying the lengthand thickness of this plate 88a,

to adiust the rate of fiow of the solid particles and of the drivingliquid by more or less reducing the cross section of flowthrough'conduits -88 and.

If it'is desired to wash the heavy elements in the centrifuging machineby means or a liquid other than the driving liquid. use can be madecones l and 8 entersblock It in the centripetal direction in the form ofan annular sheet given thereto by a piece 26, of predetermined lengthand diameter. presenting a double portion substantially conical fixed tothe summit of the centrifugal cone. The solid particles that are drivenalong pass between block N and wall 8 and are conveyed to outlet tube 4through conduit 21.

In the particular application of the invention to the decantatlon of redmud, any of the above described devices can be used. I may, forinstance, employ a centrifuging apparatus of the kind illustrated byFig. 20 including means for annular distribution or the driivng liq idsuch as shown by Fig. 10.

This centrifuging apparatus may, for instance, have the followingcharacteristics:

Speed of revolutionfrom 1200 to 1500 revoluthe decanted mud and theclarified liquidfrom 8 to 10 mms.;

Width 0 of the annular slot provided for the passage of the drivingliquid-0.6 mms.;

Width d of the annular slot provided for the e of the mud-l mm.;

Outer diameter of piece 2822 mms.

The following examples describe, by way of nonlimitative indications.applications of this apparatus to the treatment of red mud in a solutionof sodium aluminate by the Bayer process.

Example 1 After withdrawal from the autoclave and dilution, I introduceinto the centrifuging apparatus:

a. 5 cubic meters of a suspension containing 1'15 kgs. of red mud of adensity of 3.5 and 4.93 cubic meters of a solution of sodium aluminateof a density of 1.21 (25 Baum) made up of two molecules of caustic Nazoper molecule at Also:

, b. .5 cubic meters of an aluminate solution of devicesuch as shown bv8 Ac- 1; of density 1.1 (18 Baum) obtained from the washing of mudsandin which the molecular ratio of NazO to A1203 is the same.

I obtain, after treatment in the centrifuging apparatus:

1. 4.5 cubic meters of a clarified solution of sodium aluminate of adensity of 1.21 (25 Baum);

' 2. cubic meters of a suspension containing 175 kgs. of red mud and4.95 cubic meters of a solution of aluminate solution of a density of1.11 (14.28 Baum);

As the clarified aluminate solution contains 125 grs. per litre of NazO,I obtain a clear solution containing 514 kgs. of alumina for the 5 cubicmeters of suspension that have been treated.

As the solution of sodium aluminate that is utilized for attacking thebauxite contains four molecules of caustic NazO for each molecule ofA1203, I dissolve, in the treatment of bauxite, 411 kgs. of alumina perton of caustic NazO used.

Example 2 I introduce into the centrifugal apparatus:

a. 5 cubic meters of a suspension containing 325 kgs. of red mud and4.907 cubic meters of a solution of sodium aluminate of a density of1.21 (25 Baum), containing 1.4 molecules of caustic NazO per molecule ofA1203.

I obtain, after treatment in the centrifuging machine:

1. 4.07 cubic meters of a clarified solution of sodium aluminate of adensity equal to 1.21 (25 Baum);

2. 6.43 cubic meters of a. suspension containing 325 kgs. of red mudand6.337 cubic meters of a solution of aluminate of a density'of 1.115(14.85 Baum). 7

As the clarified solution of aluminate contains 125 grs. Of NazO perlitre, I obtain a clear solution containing 598 kgs. of alumina and Idissolve, in the course of the treatment of the bauxite, 764 kgs. ofalumina per ton of caustic NazO used.

Ezrample 3 I introduce into the centrifuging apparatus:

a and b. The same volumes of suspension and of driving liquid as aboveindicated in Examples 1 and 2; and

c. 0.150 cubic meter of a liquid resulting from the washing of the mud,of a density clear solution containing 604 kgs. of alumina for 5 cubicmeters of suspension treated, and I dissolve, in the course of thetreatment of the bauxite, 764 kgs. of alumina per ton of caustic NazO,as in Example 2.

Various modifications may be brought to the method of treatment setforth in the preceding examples. For instance, the suspension, onleaving the autoclave, may be introduced into the centrifuging machineeither as it is or after suitable dilution. The clear liquid that isutilized for the extraction of red mud may be replaced by water, or asuspension of red mud or of any other suitable powdery product. Thedriving liquid or suspension may also be emulsified inside the apparatusby means of a gas or vapor introduced in any suitable manner. Afterpassage through a thickening device, the suspension of red mud thatleaves the centrifuging apparatus may be introduced into a secondcentrifuging apparatus where separation and washing of the mud' willtake place. These operations may be pursued in a third centrifuginapparatus and so on. The suspension resulting from the attack of bauxitein the autoclave may,

after suitable dilution, be treated in a classifying and selectingapparatus of any suitable type with a view to separating the mud grainsof a diameter higher than a given limit (for instance 50 microns). Thesegrains may be treated separately, only the fine portion being introducedinto the centrifuging apparatus.

The mud driving liquid may be constituted of the liquor serving toattack the bauxite, in a more or less concentrated state, thus forming asuspension with the mud. This suspension, heated for a further attack,dissolved a further portion of alumina, which had remained in thebauxite after the first attack. This method of operation permits ofmethodically exhausting bauxite in several operations.

In a general manner, while I have, in the above description, disclosedwhat I deem to be practical and efficient embodiments of apparatus madeaccording to the present invention, it should be well understood that Ido not wish to be limited thereto as there might be changes made in thearrangement, disposition and form of the parts without departing fromthe principle of the present invention as comprehended within the scopeof the accompanying claims.

What I claim is:

1. A continuous method for the centrifugal separation of solid particlessuspended in a liquid comprising the steps of, introducing a suspensionof solid particles in a liquid into a centrifuge, rotating thecentrifuge and effecting the separation of the solid particles from theliquid in the suspension by centrifugal force,

- removing from the centrifuge the liquid 1 so separated, collecting theseparated solid particles at points on the inner periphery of thecentrifuge, separately introducing in a centrifugal direction a currentof auxiliary fluid into the rotating centrifuge whilst preventing itsadmixture with the suspension being centrifuged, injecting said currentof auxiliary fluid into the collected mass of separated solid particlesat the periphery by the effect of the kinetic energy acquired by themoving fluid as it travels towards the periphery, the direction ofinjection being radial with respect to the axis of rotation of thecentrifuge, entraining the separated solid particles in the current offluid so injected, and discharging by centripetal action the mixture ofauxiliary fluid and. solid particles entrained therein from thecentrifuge.

2. A method according to claim 1 wherein a gas is injected into themixture of auxiliary u d and entrained solid particles to facilitate thedischarge of the mixture from the centrifuge.

3. A method according to claim 1 in which positive pressure is appliedto the auxiliary fluid as it is introduced into the centrifuge.

4. A method according to claim 1 characterized in that the suspensioncomprises ,red mud ll in a solution of sodium aluminate as formed in theautoclave in the treatment of bauxite by the Bayer process.

5. A method according to claim 4 characterised in that the originalsuspension as obtained in the autoclave is diluted and the. auxiliaryfluid is constituted of a weak solution of sodium aluminate.

6. A process according to claim 4 characteriled in that the auxiliaryfluid is constituted of a liquor capable of attacking bauxite andfurther, in that the mixture of such liquor and entrained solidparticles discharged from the centrifuge is recycled thereto to efl'ecttreatment of further quantities of bauxite.

7. In a continuous centrifugal apparatus for separation of solidparticles from a liquid in which they are suspended, a plurality ofradiallyaxised particle collecting cones each cone in- .cluding. meansfor introducing the solid particle containing liquid, means forintroducing in a centrifugal direction an auxiliary fluid, inletdelivery tubes for said auxiliary fluid terminating at a point adjacentthe apex and peripheral walls of the cones. discharge tubes extendingfrom a ,point adjacent the apex of said cone in a radial ,centripetaldirection along the axis and through ethe entirety of said cone for saidsolid particle (containing auxiliary fluid, a removable block fltted influid-tight relationship in the apex of ,said cones, three parallelducts located within .the block in planes parallel to the axis of said.cone, means including a groove interconnecting the ends of the threeducts within the block at a point remote from the other ends of saidducts, the flrst of the three ducts connected to the inlet delivery tubefor the auxiliary fluid, the second of the three ducts openly connectedto the interior of the cone at a point adjacent the apex of the cone,and the third of the three ducts connected to the discharge tube for thesolid particle containing auxiliary fluid.

3. An apparatus according to claim 7 wherein the means interconnectingthe ends of the three ducts within the block is constituted of aremovable member and is provided with adjusting 10. In a continuouscentrifugal apparatus for separation of solid particles from a liquidsuspension thereof comprising. a rotating body member provided adjacentits outer periphery with solid particle collecting means ofsubstantially progressively decreasing cross section, means forintroducing the suspension into the body member, separate means forintroducing in a centrifugal direction an auxiliary fluid into the bodymember. said last means terminating in an outlet adjacent the smallestcross section of the collecting means whereby the auxiliary fluid issubstantially directly injected into the collected mass of separatedsolid particles. a discharge tube for said particle containing auxiliaryfluid extending from a point adjacent said outlet in a radialcentripetal direction and through the collecting means, said dischargebeing connected to an outlet on the exterior of the rotating bodymember, and independent means for removing the liquid separated from thesuspension.

11. An apparatus according to claim 10 wherein the particle collectingmeans comprise a plurality of radially-axised cones, and each cone isprovided adjacent its apex with an inlet for the auxiliary fluid andwith a discharge tube for the solid particles containing auxiliaryfluid, said tube extending along the axis of the cone.

12. An apparatus according to claim 10 wherein the discharge tube forthe particle containing auxiliary fluid is provided with an outwardlyflaring funnel-shaped inlet end. 7

13. An apparatus according to claim 10 provided with deflector meansadjacent the outlet for the auxiliary fluid in the collecting means,whereby the auxiliary fluid is injected in an annular sheet into thecollected mass of separated solid particles.

. JEAN HENRI norvrmrr.

REFERENCES CITED The following references are of record in the flle ofthis patent:

UNITED STATES PATENTS d Flowers Mar. 9, 1943

